Production of hydrogen using conventional processes such as steam reforming and coal gasification requires high temperatures on the order of 1400.degree.-1700.degree. F. and chemical catalysts. Supplying the energy to maintain these high temperature reactions, and use of these types of catalysts, is very expensive. In addition, the gaseous products of coal gasification contain heteroatoms that must be removed to prevent poisoning of the catalyst.
The production, efficient use, and recovery of hydrogen accounts for over 50 percent of the capital cost of a direct coal liquefaction plant. Hydrogen is used to increase the conversion of coal to liquid products with boiling points similar to petroleum and for hydrotreating to upgrade the quality of the liquid products. Hydrogen for direct liquefaction is expected to come from the nonliquid residue. However, the amount of residue produced from the liquefaction processes with high coal conversion may not be sufficient to provide the hydrogen requirements for the process. Mild gasification processes also produce liquids, gaseous products, and much more char than direct liquefaction processes. Therefore, careful integration of direct liquefaction and mild gasification processes will result in increased liquid yields, better economics, and better quality products because of the increased amount of hydrogen available for product upgrading. To produce hydrogen from mild gasification char and gas products, it is necessary to develop a new technology that is more efficient than conventional technologies.
Gasification of coal involves subjecting the coal to processing conditions which are sufficiently severe to bring the coal to a state at which it can be gasified. In mild gasification of coal, low temperature carbonization is performed at temperatures in the range of from 450.degree. to 750.degree. C. Mild thermal decomposition of the coal occurs, yielding "semi-coke" or "char" and "low temperature tar". The char contains considerable quantities of volatiles and complex hydrocarbons.
The char produced from mild gasification processes has a high ash and carbon content and significant amounts of heteroatoms. The hydrogen purity requirements for direct liquefaction processes are significantly lower than for most other applications. However, some components of gasification product gas such as hydrogen sulfide must be eliminated to avoid poisoning the hydrogenation catalysts used in the subsequent parts of the process.
Conventional hydrogen production methods such as steam reforming require temperatures above 900.degree. C. as well as chemical catalysts. It is not only difficult, but also expensive to provide the thermal energy required for the reaction of char with steam at high temperatures. Therefore, it would be helpful to develop a technique that does not require the use of chemical catalysts and high temperatures.
Radiofrequency energy is a noble form of energy that can be applied to enhance the rate of those reactions that can proceed only by some higher-energy free radical mechanism because the equilibrium thermodynamics are unfavorable, and those reactions that should proceed because of favorable thermodynamics, but are kinetically limited and normally require the use of a catalyst to increase the rate.
The interaction of electromagnetic fields with polar components of molecules produces free radicals and causes electrical energy to be transformed into heat. This interaction results from the response of charged components to the applied field. The displacement of these charged components from their equilibrium positions gives rise to induced dipoles that respond to the applied field. In addition to induced dipoles, some polar components contain permanent dipoles because of the asymmetric charge distribution of unlike charge partners in molecules that tend to reorient under the influence of a changing electric field. This charge gives rise to orientation polarization. Finally, another source of polarization arises from charge buildup in interfaces between components in heterogeneous systems. These are termed interfacial, space-charged, or Maxwell-Wagner polarization. These two mechanisms are the basis of radiofrequency energy processes. The lag between the zero crossing of the applied field and relaxation of molecules causes a conversion of radiofrequency energy into heat by friction between molecules. The lag is called the relaxation time, and is widely used by physical chemists to study structures of molecules. Relaxation times may be 1.times.10.sup.-6 seconds for large molecules such as polymers, and 1.times.10.sup.-12 sec. for small molecules such as water. Relaxation times are slow for hindered molecules like solids and viscous liquids, and are fast for free moving liquids and gases.
Helm, Jr., in U.S. Pat. No. 4,435,374, discloses a method for gasifying solid carbonaceous material to form carbon monoxide and hydrogen by contacting the material with superheated steam at at least 650.degree. C. and irradiating the product of this contacting with an amount of microwave energy sufficient to gasify the carbon.
Waltrip, in U.S. Pat. No. 3,480,529, discloses a method for dissociating chemical elements from a medium containing these elements using an audiofrequency harmonic tone having a predetermined rate of vibration selected on the basis of the element to be manipulated. This method is primarily directed to desalinating water and purifying sewage.
Zavitsanos et al., in U.S. Pat. No. 4,076,607, disclose a method of coal desulfurization wherein the coal is irradiated with microwave energy which rearranges the chemical bonds between the sulfur and other compounds in the coal. Sulfur is liberated in the form of at least one stable gaseous compound, such as hydrogen sulfide, sulfur dioxide, or sulfur carbonyl.
Knoevenagel et al., in U.S. Pat. No. 3,977,952, disclose a method for decomposing carbon-containing compounds by subjecting compositions containing these compounds to radiation of a wavelength of about 20 to 600 nm in the presence of water and oxygen.
Bodine, Jr., in U.S. Pat. No. 2,745,861, discloses the use of sound waves to produce carbon monoxide, synthesis gases, and synthetic hydrocarbons from combustible raw materials such as powdered coal, coke, wood flour, etc.
Hodge, in U.S. Pat. No. 2,542,028, discloses the use of high-frequency apparatus for the controlled heating of carbonaceous minerals to form hydrocarbon oils and distilling the hydrocarbon oils thus produced.
Rafflower, in U.S. Pat. No. 1,624,625, discloses a process for desulfurizing coal, water, or mixed gases by contacting the material to be desulfurized with ferric oxide, which immediately reacts with the sulfur.
Kruesi et al., in U.S. Pat. No. 4,311,520, disclose the use of microwave energy as the source of heat for recovering nickel, cobalt, or manganese from their oxides or silicates.